6 Conclusions

The applied method provides plausible values for the coronal Alfvén velocity and magnetic field. The data scatter is much larger than the errors of measurement thus indicating a diversity of densities and field strengths in the corona. The magnetic field strength decreases from 1-7 G at $R\approx 1.6$ to 0.3-0.9 G at $R\approx 2.5$. Such a result is in a good agreement with other estimates at this height range, as well as with the B=5H^-1.5 relationship proposed by Dulk & McLean (1978). When placed in the wider context together with the data covering the 1<R<10 range, the data indicate that below the height of $H\approx 0.3$ the magnetic field is governed by the active region fields. Above $H\approx 1$ the radial field, decreasing as R^-2, becomes dominant.

The results show that the coronal value of the parameter $\beta$is around $\beta\approx 0.1$, slowly increasing with the height. This is consistent with the generally decreasing trend of the Alfvén velocity (Fig. 3a) since the coronal temperature can be considered as approximately constant. Beside the overall decreasing trend, the data indicate that there is a local minimum of the Alfvén velocity of $v_{\rm A}\approx$ 400-500 km s^-1at $R\approx$ 1.7-2.2 and a local maximum of $v_{\rm A}\approx$ 450-700 km s^-1 at $R\approx$ 1.8-2.5, depending on the coronal density model used. Such behaviour of the Alfvén velocity is qualitatively similar to that anticipated by Mann et al. (2002).

The inferred shock velocities decrease, whereas the density jumps increase in the region of $v_{\rm A}={\rm min}$. Most of the dm-m type II bursts start at heights lower or close to the Alfvén velocity minimum and cease close to the heights where it attains the maximum.

Finally, we note that the frequency drift of dm-m type II bursts depends on the emission frequency as a power-law $D_f\propto f_{\rm L}^{1.9}$, very similar to that found by Vrsnak et al. (2001b) in IP type II bursts.

Acknowledgements

We are grateful to the Culgoora Solar Observatory for the open data policy. We are thankful to Dr. P. Zlobec for helpful comments and suggestions. J.M. acknowledges a grant under MIUR Cofin.